1. Field of the Invention
The present invention relates to a method for controlling a fuel cell system.
2. Discussion of the Background
A fuel cell system acquires DC electric energy according to an electrochemical reaction of a fuel gas (gas essentially containing hydrogen, such as hydrogen gas) and an oxide gas (gas essentially containing oxygen, such as air) respectively supplied to an anode electrode and a cathode electrode. This system is of a stationary type, or is mounted in a fuel cell vehicle as an on-vehicle fuel cell system.
For example, a solid polymer fuel cell has an electrolyte membrane/electrode assembly (MEA) having an anode electrode and a cathode electrode provided on the respective side of an electrolyte membrane formed by a polymer ion-exchange film; the electrolyte membrane/electrode assembly is sandwiched by a pair of separators. A fuel gas passage for supplying a fuel gas to the anode electrode is formed between one of the separators and the electrolyte membrane/electrode assembly. An oxide gas passage for supplying an oxide gas to the cathode electrode is formed between the other separator and the electrolyte membrane/electrode assembly.
When the fuel cell is stopped, supply of the fuel gas and oxide gas is stopped. However, the fuel gas remains in the fuel gas passage, and the oxide gas remains in the oxide gas passage. When the operation-stop period of the fuel cell becomes long, therefore, the fuel gas and the oxide gas may pass through the electrolyte membrane, so that the fuel gas is mixed with the oxide gas to react therewith, thereby deteriorating the electrolyte membrane/electrode assembly.
To cope with the problem, a fuel cell system disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2004-22487 (FIG. 1 and paragraph [0029]) shuts off the supply of a reaction gas to the anode side, and shuts off the supply of the reaction gas to the cathode side from a blower (air pump) when the operation of the fuel cell is stopped. Further, the exhaust gas on the anode side is circulated to the upstream side through an anode-side circulation line, and the exhaust gas on the cathode side is circulated to the upstream side through a cathode-side circulation line, so that an electrochemical reaction in the fuel cell is maintained to thereby charge the battery with the generated power. Further, hydrogen in the exhaust gas on the anode side is consumed and oxygen in the exhaust gas on the cathode side is consumed this way, and a nitrogen gas is stored in a tank. The gases in the anode and cathode of the fuel cell are replaced with the nitrogen gas stored in the tank.
The replacement of the gases in the anode and cathode of a fuel cell with an inactive gas like the nitrogen gas when the operation of the fuel cell is stopped as done according to the technique described in Japanese Unexamined Patent Application Publication No. 2004-22487 reduces the possibility of causing an unnecessary reaction after the operation is stopped. This effect is preferable from the viewpoint of preventing degradation of the fuel cell.
However, the fuel cell system disclosed in Japanese Unexamined Patent Application Publication No. 2004-22487 needs the cathode-side circulation line, the tank to store a nitrogen gas, and a line for supplying the nitrogen gas to the anode side, which complicates the configuration of the fuel cell system and makes the fuel cell system expensive.
As a possible solution to this problem, the operation of the fuel cell may be stopped in such a way that supply of the fuel gas to the anode side is shut off, and the amount of air supplied to the cathode side is reduced to maintain power generation so that the cathode side is filled with nitrogen.
When such a process with the operation of the fuel cell stopped is executed, the anode side is kept airtight with negative pressure increased. Since the cathode side communicates with the atmosphere via the air pump, the pressure on the cathode side becomes atmospheric pressure (0 kPg). During soaking, therefore, an inter-electrode differential pressure between the anode and the cathode becomes large.
In the fuel cell system, an air inlet passage is provided between a fuel-gas inlet hole on the anode side and the outlet side of the air pump with an air inlet valve disposed in the air inlet passage to blow off (scavenge) water or the like staying on the anode side out of the fuel cell stack when, for example, the outside temperature is likely to fall below the freezing point during soaking.
In this case, a large-capacity discharge valve is provided in parallel to a purge valve between an off-gas passage communicating with a fuel-gas outlet hole on the anode side and the entrance of a dilution box. In executing the scavenging process (referred to as “anode scavenging process” or “anode-air scavenging process”), the air inlet valve is opened, and the discharge valve (purge valve if needed) is opened to let compressed air from the air pump flow into the dilution box through the air inlet valve, the anode in the fuel cell and the discharge valve (purge valve). The outlet side of the dilution box communicates with the atmosphere. Blowing liquid droplets or the like on the anode side this way prevents freezing on the anode side.
A normally-closed on-off solenoid valve (which (or whose valve body) is closed when not energized) which (or whose valve body) is closed by elastic force and is opened by electromagnetic force is used for the foregoing air inlet valve, purge valve and discharge valve.
Therefore, the aforementioned large inter-electrode differential pressure is applied to the air inlet valve, purge valve and discharge valve before the anode scavenging process is executed at the time of soaking after the operation of the fuel cell system is stopped.
At the time the fuel cell system according to the related art is started, as illustrated in a timing chart in FIG. 12, when the ignition switch (operation switch) is set on (IG ON), the air pump is driven to supply the oxide gas to the cathode, and when a time Ts during which the cathode pressure rises to a predetermined pressure Pd according to an increase in the number of rotations of the air pump elapses, the hydrogen shutoff valve is opened to supply a high-pressure fuel gas.